U.S. patent application number 10/901901 was filed with the patent office on 2005-02-03 for adaptive modulation and coding.
Invention is credited to Awad, Yassin Aden, Vadgama, Sunil Keshavji.
Application Number | 20050025254 10/901901 |
Document ID | / |
Family ID | 27799579 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050025254 |
Kind Code |
A1 |
Awad, Yassin Aden ; et
al. |
February 3, 2005 |
Adaptive modulation and coding
Abstract
In an adaptive modulation and coding method one or more
adjustable values are created (S1), each corresponding to at least
one of a plurality of available modulation and coding levels
applicable to a signal transmitted from a transmitter to a
receiver, and each representing a change to the level(s) to which
it corresponds. One or more of said adjustable values is/are
adjusted in dependence upon whether or not the signal is received
successfully by the receiver (S2-S5). One of said available
modulation and coding levels is selected (S6-S8) to apply to the
signal based on such an adjustable value. Such a method can enable
the appropriate modulation and coding level to be selected even
when the path and channel conditions vary. The method is applicable
to selecting modulation and coding levels in a high-speed downlink
packet access system of a wireless communication network.
Inventors: |
Awad, Yassin Aden;
(Southhall Middlesex, GB) ; Vadgama, Sunil Keshavji;
(Ashford Middlesex, GB) |
Correspondence
Address: |
KATTEN MUCHIN ZAVIS ROSENMAN
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
27799579 |
Appl. No.: |
10/901901 |
Filed: |
July 29, 2004 |
Current U.S.
Class: |
375/295 |
Current CPC
Class: |
H04L 1/0009 20130101;
H04L 1/0003 20130101; H04L 1/203 20130101; H04L 1/0021
20130101 |
Class at
Publication: |
375/295 |
International
Class: |
H04L 027/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2003 |
GB |
0317968.6 |
Claims
What we claim is:
1. An adaptive modulation and coding method comprising: holding one
or more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
signal transmitted from a transmitter to a receiver, and each
representing a change to the or each said level to which it
corresponds; adjusting one or more of said adjustable values in
dependence upon whether or not the signal is received successfully
by the receiver; and selecting one of said available modulation and
coding levels to apply to said signal based on such an adjustable
value.
2. A method as claimed in claim 1, wherein the holding, adjusting
and selecting steps are carried out by the transmitter.
3. A method as claimed in claim 1, wherein the receiver transmits
to the transmitter an indication of whether or not the signal was
received successfully, and the transmitter adjusts one or more of
the adjustable values in dependence upon the received
indication.
4. A method as claimed in claim 1, wherein the adjusting step
comprises increasing one or more of the adjustable values when the
signal is received successfully and decreasing one or more of said
adjustable values when the signal is not received successfully.
5. A method as claimed in claim 1, wherein there are at least two
said adjustable values and said adjusting step comprises adjusting
one said adjustable value by an amount different from an amount by
which another said adjustable value is adjusted.
6. A method as claimed in claim 1, wherein an amount by which the
or one said adjustable value is adjusted is dependent upon a target
error rate.
7. A method as claimed in claim 1, wherein the or each said
adjustable value is a non-integer value and in the selecting step a
rounded version of the or one said adjustable value is employed to
select said modulation and coding level to apply to said signal,
said rounded version representing the nearest integer value to said
non-integer adjustable value.
8. A method as claimed in claim 1, further comprising: proposing
one of the available modulation and coding levels based on a signal
transmission quality; and selecting said modulation and coding
level based on the adjustable value corresponding to the proposed
modulation and coding level.
9. A method as claimed in claim 8, wherein the receiver produces a
measure of said signal transmission quality, and the method further
comprises employing a fixed mapping to map said measure to the
proposed modulation and coding level.
10. A method as claimed in claim 8, wherein the proposing step is
carried out by the receiver which transmits information specifying
the proposed modulation and coding level to the transmitter.
11. A method as claimed in claim 1, wherein one or said transmitter
and said receiver is a base station of a wireless communication
system, and the other of said transmitter and said receiver is a
user equipment of said system.
12. A method as claimed in claim 11, wherein said signal is a
packet access signal.
13. Adaptive modulation and coding apparatus comprising: an
adjustable value holding unit which holds one or more adjustable
values, each corresponding to at least one of a plurality of
available modulation and coding levels applicable to a signal
transmitted from a transmitter to a receiver, and each representing
a change to the or each said level to which it corresponds; an
adjusting unit which adjusts one or more of said adjustable values
in dependence upon whether or not the signal is received
successfully by the receiver; and a selecting unit which selects
one of said available modulation and coding levels to apply to said
signal based on such an adjustable value.
14. A transmitter, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
signal transmitted from the transmitter to a receiver of said
system, and each representing a change to the or each said level to
which it corresponds; an adjusting unit which adjusts one or more
of said adjustable values in dependence upon whether or not said
signal is received successfully by said receiver; and a selecting
unit which selects one of said available modulation and coding
levels to apply to said signal based on such an adjustable
value.
15. A receiver, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
signal transmitted from a transmitter of said system to said
receiver, and each representing a change to the or each said level
to which it corresponds; an adjusting unit which adjusts one or
more of said adjustable values in dependence upon whether or not
said signal is received successfully by said receiver; a selecting
unit which selects one of said available modulation and coding
levels based on such an adjustable value; and a level informing
unit which transmits to said transmitter information specifying
said selected modulation and coding level.
16. A base station, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
downlink signal transmitted from the base station to a user
equipment of said system, and each representing a change to the or
each said level to which it corresponds; an adjusting unit which
adjusts one or more of said adjustable values in dependence upon
whether or not said downlink signal is received successfully by
said user equipment; and a selecting unit which selects one of said
available modulation and coding levels to apply to said downlink
signal based on such an adjustable value.
17. A base station, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to
an uplink signal transmitted from a user equipment of said system
to said base station, and each representing a change to the or each
said level to which it corresponds; an adjusting unit which adjusts
one or more of said adjustable values in dependence upon whether or
not said uplink signal is received successfully by said base
station; a selecting unit which selects one of said available
modulation and coding levels based on such an adjustable value; and
a level informing unit which transmits to said user equipment
information specifying said selected modulation and coding
level.
18. A user equipment, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to
an uplink signal transmitted from the user equipment to a base
station of said system, and each representing a change to the or
each said level to which it corresponds; an adjusting unit which
adjusts one or more of said adjustable values in dependence upon
whether or not said uplink signal is received successfully by said
base station; and a selecting unit which selects one of said
available modulation and coding levels to apply to said uplink
signal based on such an adjustable value.
19. A user equipment, for use in a wireless communication system,
comprising: an adjustable value holding unit which holds one or
more adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
downlink signal transmitted from a base station of said system to
said user equipment, and each representing a change to the or each
said level to which it corresponds; an adjusting unit which adjusts
one or more of said adjustable values in dependence upon whether or
not said downlink signal is received successfully by said user
equipment; a selecting unit which selects one of said available
modulation and coding levels based on such an adjustable value; and
a level informing unit which transmits to said base station
information specifying said selected modulation and coding
level.
20. A machine-readable recording medium having recorded thereon an
operating program which, when run on a processor in a transmitter
of a wireless communication system, causes the transmitter to carry
out the steps of: holding one or more adjustable values, each
corresponding to at least one of a plurality of available
modulation and coding levels applicable to a signal transmitted
from said transmitter to a receiver of said system, and each
representing a change to the or each said level to which it
corresponds; adjusting one or more of said adjustable values in
dependence upon whether or not the signal is received successfully
by said receiver; and selecting one of said available modulation
and coding levels to apply to said signal based on such an
adjustable value.
21. A machine-readable recording medium having recorded thereon an
operating program which, when run on a processor in a receiver of a
wireless communication system, causes the receiver to carry out the
steps of: holding one or more adjustable values, each corresponding
to at least one of a plurality of available modulation and coding
levels applicable to a signal transmitted from a transmitter of
said system to said receiver, and each representing a change to the
or each said level to which it corresponds; adjusting one or more
of said adjustable values in dependence upon whether or not the
signal is received successfully by said receiver; selecting one of
said available modulation and coding levels based on such an
adjustable value; and transmitting to said transmitter information
specifying the selected modulation and coding level.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to adaptive modulation and
coding methods and apparatus for use, for example, in wireless
communication systems.
[0003] 2. Description of the Related Art
[0004] FIG. 1 shows parts of a wireless communication system 1. The
system includes a plurality of base stations 2, only one of which
is shown in FIG. 1. The base station 2 serves a cell in which a
plurality of individual users may be located. Each user has an
individual user equipment (UE). Only the user equipments UE2, UE11
and UE50 are shown in FIG. 1. Each UE is, for example, a portable
terminal (handset) or portable computer.
[0005] As is well known, in a code-division multiple access (CDMA)
system the signals transmitted to different UEs from the base
station (also known as "node B") are distinguished by using
different channelisation codes. In so-called third generation
wireless communication systems a high speed downlink packet access
(HSDPA) technique has been proposed for transmitting data in the
downlink direction (from the base station to the UEs). In this
technique a plurality of channels are available for transmitting
the data. These channels have different channelisation codes. For
example, there may be ten different channels C1 to C10 available
for HSDPA in a given cell or sector of a cell. In HSDPA, downlink
transmissions are divided up into a series of transmission time
intervals (TTI) or frames, and a packet of data is transmitted on
each different available channel to a selected UE. A new choice of
which UE is served by which channel can be made in each TTI.
[0006] FIG. 2 shows an example of the operation of the HSDPA
technique over a series of transmission time intervals TTI1 to
TTI9. As shown in FIG. 2, in TTI1 it is determined that two packets
will be sent to UE50, four packets will be sent to UE11 and four
packets will be sent to UE2. Accordingly, two channels are
allocated to UE50 and four channels each are allocated to UE11 and
UE2. Thus, as shown in FIG. 1, UE50 is allocated channels C1 and
C2, UE11 is allocated channels C3 to C6, and UE2 is allocated
channels C7 to
[0007] In the next transmission time interval TTI2 a new user
equipment UE1 is sent one packet, and the remaining UEs specified
in TTI1 continue to receive packets.
[0008] Thus, effectively the HSDPA system employs a number of
parallel shared channels to transmit data in packet form from the
base station to the different UEs. This system is expected to be
used, for example, to support world wide web (WWW) browsing.
[0009] In the HSDPA system, channel state information (CSI) is made
available to both the transmitter and the receiver, in order to
realise a robust communication system structure. The HSDPA system
is intended to increase the transmission rates and throughput, and
to enhance the quality of service (QoS) experienced by different
users. It transfers most of the functions from the base station
controller (also known as the radio network controller or RNC) to
the base transceiver station (node B).
[0010] The HSDPA system may also use a control technique referred
to as an adaptive modulation and coding scheme (AMC) to enable the
base station to select different modulation and/or coding schemes
under different channel conditions.
[0011] The signal transmission quality for a channel between the
transmitter and a receiver (UE) varies significantly over time.
FIG. 3 shows an example of the variation of a
signal-to-interference ratio (SIR) a downlink channel for four
different users over a series of 5000 TTIs. This plot was obtained
by a simulation. As illustrated, for a given UE the range of SIR
values may be as much as from around +12 dB to -15 dB. The SIR
value varies due to shadowing, Rayleigh fading, and change in
distribution of the mobile UEs, as well as cellular area
specifications including the propagation parameters and speeds of
UEs.
[0012] FIG. 4 is a graph illustrating a relationship between a data
transmission rate (throughput) and signal-to-interference ratio for
four different modulation and coding combinations, also referred to
as modulation-and-coding scheme (MCS) levels. The first three
levels (MCS8, MCS6 and MCS5) are all quadrature amplitude
modulation (QAM) schemes which differ from one another in the
number (64 or 16) of constellation points used. The fourth level
(MCS1) uses quadrature phase shift keying (QPSK) as its modulation
scheme.
[0013] Each level uses coding defined by a coding parameter which,
in this example, is expressed as a redundancy rate R. For the first
two levels MCS8 and MCS6 the redundancy rate R is 3/4, and for the
third and fourth levels MCS5 and MCS1 the redundancy rate is
1/2.
[0014] As can be seen from FIG. 4, for SIR values lower than around
-4 dB MCS1 (QPSK, R=1/2) is the best available option. The
characteristic of this level is plotted with circles in the
figure.
[0015] For SIR values in the range from around -4 dB to around +2
dB, MCS5 (16QAM, R=1/2) provides the best transmission rate. The
characteristic for this MCS level is illustrated by crosses in the
figure.
[0016] For SIR values between around +2 dB and +8 dB MCS6 (16QAM,
R=3/4) provides the best transmission rate. The characteristic for
this MCS level is illustrated by diamond points in the figure.
[0017] Finally, for SIR values greater than around +8 dB, MCS8 (64
QAM, R=3/4) provides the best transmission rate. The characteristic
of this combination is illustrated by square points in the
figure.
[0018] In the HSDPA system a technique such as adaptive modulation
and coding (AMC) is used to adapt the MCS level in accordance with
the variations of the channel condition (e.g. SIR value).
[0019] According to the HSDPA standard (3GPP TS 25.214 V5.5.0
(2003-6)), each UE holds a channel quality indicator (CQI) mapping
table. An example of the mapping table is shown in FIG. 5. As the
table shows, for each CQI value various parameters are defined
including a transport block size, a number of codes, a modulation
type, and a reference power adjustment .DELTA.. The transport block
size represents a maximum number of bits which can be received in
one TTI. The number of codes is the number of channelisation codes
which are sent simultaneously to a single user within one TTI. The
modulation type represents the type of modulation scheme, eg QPSK
or 16QAM. The reference power adjustment .DELTA. is a reduction to
be applied to the transmitted power if the transmitted power is
greater than that necessary for the signal to be receivable at the
CQI value.
[0020] Each UE produces a measure of the quality of a downlink
channel from the base station to the UE. Based on this measure and
on the CQI mapping table the UE reports the highest CQI value for
which a signal having the transport block size, number of codes and
modulation for that value is receivable with a transport block
error probability (also referred to as a Packet Error Rate (PER))
below a certain target value.
[0021] There may be a one-to-one correspondence between the CQI
values and MCS levels, so that if desired the base station may
directly take the reported CQI value as the MCS level to apply. For
example, in one proposal (3GPP TSGR1-02-0459, "HSDPA CQI proposal",
9-12 Apr. 2002, Paris, France), there are CQI values 1 to 30 which
are intended to provide approximately a 1 dB step size between
adjacent MCS levels at 10% PER. Alternatively, the base station may
employ the reported CQI value for each UE, as well as information
relating to the system limitations and available MCS levels, to
identify the most efficient MCS level for the particular UE.
[0022] Thus, based on the reported CQI values, UEs that have better
channels or are located in the vicinity of the base station can
employ higher levels of MCS and therefore enjoy higher transmission
rates. Effectively, the result is a classification of the
transmission rates based on the channel quality of each UE.
[0023] Ideally, each UE reports a CQI value in every TTI and the
base station is capable of setting a new MCS level for each
available channel in every TTI.
[0024] The HSDPA system may also employ a hybrid automatic repeat
request (H-ARQ) technique.
[0025] FIG. 6 is a schematic diagram for use in explaining how the
H-ARQ technique works. In this example, the technique is a
so-called stop-and-wait (SAW) version of the technique. The figure
shows packet transmissions in a single downlink channel HSPDSCH1
over a series of successive TTIs, TTI1 to TTI9. In TTI2 a first
packet is transmitted to UE1. Upon receiving a packet, each UE
checks whether the transmission was error-free. If so, the UE sends
an acknowledge message ACK back to the base station using an uplink
control channel such as the dedicated physical control channel
(DPCCH). If there was an error in the transmission of the received
packet, the UE sends a non-acknowledge message NACK back to the
base station using the uplink channel.
[0026] In the example shown in FIG. 6, the first packet transmitted
to UE1 in TTI2 fails to be received error-free, and accordingly
some time later, in TTI4, UE1 sends the NACK message to the base
station. In the H-ARQ technique it is permitted for the next packet
destined for a particular UE to be transmitted without waiting for
the acknowledge or non-acknowledge message of a packet previously
transmitted to the same UE. Thus, none of the transmission
timeslots can go idle in the case of error-free channels, which
gives the ability to schedule UEs freely. System capacity is saved
while the overall performance of the system in terms of delivered
data is improved.
[0027] For example, as shown in FIG. 6, before the NACK message for
the first packet of UE1 is received by the base station, the base
station transmits a second packet to UE1 in TTI4. Thus, this second
packet for UE1 is transmitted before the first packet for UE1 is
retransmitted in TTI7 in response to the NACK message for the first
transmission of the first packet.
[0028] In the H-ARQ technique, an erroneously-received packet
(failed packet) is subject to a so-called chase combining process.
In this process a failed packet is resent by the transmitter and
subsequently the receiver "soft" combines (for example using
maximal ratio combining) all received copies of the same packet.
The final SIR is determined as the sum of the respective SIRs of
the two packets being combined. Thus, the chase combining process
improves the SIR of the transmitted packets.
[0029] Further information regarding AMC and HARQ techniques may be
found in 3GPP TR 25.848 V 4.0.0 (2001-03), Third Generation
Partnership Project; Technical Specification Group Radio Access
Network; Physical Layer Aspects of UTRA High Speed Downlink Packet
Access (release 4), March 2001, the entire content of which is
incorporated herein by reference.
[0030] The switching between different MCS levels has been
recognised as a very critical task, and recently there have been
various proposals for optimising this switching. For example, in
TSG R1-1-0589, TSG-RAN Working Group 1 meeting no. 20, Busan,
Korea, May 21 to 25, 2001, NEC and Telecom MODUS jointly proposed
an AMC technique in which the thresholds for switching between
different MCS levels are adjusted based on the ACK/NACK signalling
from the UE. If NACK is signalled, the base station increases the
thresholds by an upward amount S1. If ACK is signalled, the base
station decreases the thresholds by a downward amount S2. The
adjustments to the thresholds are limited and, for simplicity, the
differences between thresholds may be fixed. The ratio between the
upward amount S1 and the downward amount S2 may be determined based
on the target error rate.
[0031] This AMC method adjusts the thresholds between MCS levels to
try to take into account different operating conditions in the
wireless communication system. In particular, the optimum MCS
levels under any particular signal conditions depend on the Doppler
frequency (i.e. the speed at which the UE is moving) and the
multi-path propagation conditions. For example, FIG. 7 shows the
effect of the UE speed on the throughput-vs.-SIR characteristic for
each of the different MCS levels in FIG. 4. Three lines are plotted
per MCS level: the highest line corresponds to a low UE speed of 3
km/h (Doppler frequency Fd=5.555 Hz), the middle line corresponds
to a medium UE speed of 60 km/h (Fd=111.112 Hz), and the lowest
line corresponds to a high UE speed of 120 km/h (Fd=222.24 Hz).
FIG. 7 shows that throughput declines as UE speed increases. It can
also be seen that the optimum thresholds for switching between MCS
levels are also changed as the UE speed changes.
[0032] FIG. 7 relates to a single-path Rayleigh fading mode. FIG. 8
shows the effect of different UE speeds under path conditions of
two equal-gain paths. It can be seen that the characteristics are
very different from FIG. 6, and it is clear that the optimum
thresholds are changed as the path conditions change.
[0033] The method proposed by NEC/Telecom MODUS changes the
thresholds as the operating conditions change but the method does
not provide a satisfactory solution as it increases or decreases
the threshold each time an ACK or NACK message is received, i.e.
every frame. When the step size between thresholds for switching
MCS levels is significant (eg a few dB) this appears to result in
relatively poor performance at lower MCS levels for path conditions
in which there is effectively a single dominant path, for example
in open countryside.
[0034] In another AMC method proposed by NEC and Telecom MODUS in
TSG R1-1-0589 the base station selects a MCS level based on the
ACK/NACK signalling from the UE. For example, the base station
lowers the MCS level if NACK is received, and increases the MCS
level if ACK is received successively for a certain number of TTIs.
This method has the advantage that it does not rely on results of
measuring the channel quality to select the MCS levels. Thus,
problems of measurement accuracy and reporting delay are avoided.
However, this method appears to have relatively poor performance at
high SIR values when there are two paths of comparable strength,
for example in an urban environment.
SUMMARY OF THE INVENTION
[0035] According to a first aspect of the present invention there
is provided an adaptive modulation and coding method. The method
comprises holding one or more adjustable values, each corresponding
to at least one of a plurality of available modulation and coding
levels applicable to a signal transmitted from a transmitter to a
receiver, and each representing a change to the or each level to
which it corresponds. One or more of the adjustable values is/are
adjusted in dependence upon whether or not the signal is received
successfully by the receiver. One of the available modulation and
coding levels is selected to apply to the signal based on such an
adjustable value.
[0036] Such an adaptive modulation and coding method can enable the
appropriate modulation and coding level to be selected even when
the path and channel conditions vary.
[0037] In one preferred embodiment, the holding, adjusting and
selecting steps are carried out by the transmitter. Alternatively,
however, the holding, adjusting, and selecting steps may be carried
out by the receiver, with the receiver informing the transmitter of
the selected modulation and coding level.
[0038] When the adjusting step is carried out in the transmitter,
the receiver may transmit to the transmitter an indication of
whether or not the signal was received successfully. The indication
may be, for example, an ACK/NACK signal. The transmitter then
adjusts one or more of the adjustable values in dependence upon the
received indication.
[0039] In one embodiment, the adjustable values are held in a table
which stores adjustable values corresponding respectively to all of
the plurality of available modulation and coding levels. The table
may be "seeded" by simply making each adjustable value equal to its
corresponding one of the available modulation and coding levels.
Thereafter, the adjustable values may be adjusted in dependence
upon whether or not the signal is received successfully by the
receiver.
[0040] With such a table, it is possible for each one of the
adjustable values to be adjusted by a different amount, if desired,
in the adjusting step. Alternatively, it is possible to adjust only
some of the adjustable values in the adjusting step whilst leaving
other values unchanged. Also, because all the adjustable values are
held in the table, retrieval and updating of the values can be
quick and efficient. For example, in a preferred embodiment, the
receiver proposes one of the available modulation and coding levels
based on a signal transmission quality, and the final modulation
and coding level is selected based on the adjustable value
corresponding to the proposed modulation and coding level. In this
case, the proposed modulation and coding level can be used as an
index to the table to simplify the selection of the corresponding
adjustable value.
[0041] It is not necessary to use a table to hold the adjustable
values. In another embodiment, one or more shared adjustable values
are held. The or each adjustable value may correspond to more than
one of the plurality of available modulation and coding levels. For
example, one group of available modulation and coding levels
sharing the same basic modulation type (e.g. QPSK) may have the
same corresponding adjustable value, whereas another such group
(e.g. levels having another modulation type such as 16QAM) may have
another corresponding adjustable value. In this case, the or each
adjustable value may be an offset value, with the final modulation
and coding level being selected by applying the offset value to the
modulation and coding level proposed by the receiver. The or each
adjustable value may be set to 0 in an initialisation step, and
thereafter be subject to adjustment in dependence upon whether or
not the signal is received successfully by the receiver.
[0042] If the transmitter and receiver operate repetitively, for
example on a time slot by time slot or frame by frame basis, then
the adjusting and selection steps may be carried out per time slot
or per frame. In this way, based on whether or not the signal was
received successfully by the receiver in one time slot or frame,
the adjustable values may be adjusted and a new modulation and
coding level selected to apply to the signal transmitted in the
next time slot or frame.
[0043] In one embodiment, the adjusting step comprises increasing
one or more of the adjustable values when the signal is received
successfully and decreasing one or more of the adjustable values
when the signal is not received successfully.
[0044] In the adjusting step, at least one of the adjustable values
may be increased by an amount different from an amount by which
another one of the corresponding adjustable values is adjusted.
This can be useful if, for example, the modulation and coding
levels comprise two or more groups of levels such as one group of
levels for QPSK modulation and another group of levels for 16QAM
modulation. In this case, the adjustment amounts applied to one
group may be made different from those applied to the other
group.
[0045] Preferably, the or each adjustable value is a non-integer
value. In the selecting step, a rounded version of the adjustable
value may be employed to select the modulation and coding level to
apply to the signal, the rounded version representing the nearest
integer value to the non-integer adjustable value. In this way,
although it is of course necessary to produce an integer value for
the final selected modulation and coding level, the adjustable
values can be maintained with a higher precision so that the most
appropriate modulation and coding level is selected based on the
prevailing path and channel conditions.
[0046] The receiver may produce a measure of signal transmission
quality, for example a signal-to-interference and noise ratio. A
fixed mapping may then be employed, either in the receiver or in
the transmitter, to map this measure to a proposed modulation and
coding level. For example, in one embodiment, the receiver produces
a CQI value which it reports to the transmitter. This CQI value may
have a one-to-one correspondence with the available MCS levels. In
this case, the reported CQI value is used by the transmitter to
select the adjustable value, and the selected adjustable value is
then used to derive the final modulation and coding level.
[0047] The method may be used in a wireless communication system,
in which case the transmitter may be a base station and the
receiver may be a user equipment of the wireless communication
system. The signal transmitted from the base station to the user
equipment may be a downlink packet access signal. Alternatively,
the transmitter may be a user equipment and the receiver may be a
base station, and the signal transmitted from the user equipment to
the base station may be an uplink packet access signal.
[0048] According to a second aspect of the present invention there
is provided adaptive modulation and coding apparatus. The apparatus
comprises an adjustable value holding unit which holds one or more
adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
signal transmitted from a transmitter to a receiver, and each
representing a change to the or each level to which it corresponds.
The apparatus also comprises an adjusting unit which adjusts one or
more of the adjustable values in dependence upon whether or not the
signal is received successfully by the receiver. A selecting unit
selects one of the available modulation and coding levels to apply
to the signal based on such an adjustable value.
[0049] According to a third aspect of the invention there is
provided a transmitter for use in a wireless communication system.
The transmitter comprises an adjustable value holding unit which
holds one or more adjustable values, each corresponding to at least
one of a plurality of available modulation and coding levels
applicable to a signal transmitted from the transmitter to a
receiver of said system, and each representing a change to the or
each level to which it corresponds. An adjusting unit adjusts one
or more of the adjustable values in dependence upon whether or not
the signal is received successfully by the receiver. A selecting
unit selects one of the available modulation and coding levels to
apply to the signal based on such an adjustable value.
[0050] The transmitter may be part of a base station or part of a
user equipment of a wireless communication system.
[0051] According to a fourth aspect of the present invention there
is provided a receiver for use in a wireless communication system.
The receiver comprises an adjustable value holding unit which holds
one or more adjustable values, each corresponding to at least one
of a plurality of available modulation and coding levels applicable
to a signal transmitted from a transmitter of the system to the
receiver, and each representing a change to the or each level to
which it corresponds. An adjusting unit adjusts one or more of the
adjustable values in dependence upon whether or not the signal is
received successfully by the receiver. A selecting unit selects one
of the available modulation and coding levels based on such an
adjustable value. A level informing unit transmits to the
transmitter information specifying the selected modulation and
coding level.
[0052] The receiver may be part of a base station or part of a user
equipment of a wireless communication system.
[0053] According to a fifth aspect of the present invention there
is provided an operating program which, when run on a processor in
a transmitter of a wireless communication system, causes the
transmitter to carry out the steps of: holding one or more
adjustable values, each corresponding to at least one of a
plurality of available modulation and coding levels applicable to a
signal transmitted from the transmitter to a receiver of the
system, and each representing a change to the or each level to
which it corresponds; adjusting one or more of the adjustable
values in dependence upon whether or not the signal is received
successfully by the receiver; and selecting one of the available
modulation and coding levels to apply to the signal based on such
an adjustable value.
[0054] According to a sixth aspect of the present invention there
is provided an operating program which, when run on a processor in
a receiver of a wireless communication system, causes the receiver
to carry out the steps of: holding one or more adjustable values,
each corresponding to at least one of a plurality of available
modulation and coding levels applicable to a signal transmitted
from a transmitter of the system to the receiver, and each
representing a change to the or each level to which it corresponds;
adjusting one or more of the adjustable values in dependence upon
whether or not the signal is received successfully by the receiver;
selecting one of the available modulation and coding levels based
on such an adjustable value; and transmitting to the transmitter
information specifying the selected modulation and coding
level.
[0055] In the fifth and sixth aspects of the invention, one of the
transmitter and the receiver may be part of a base station of a
wireless communication system, and the other of the transmitter and
the receiver may be part of a user equipment of the system.
[0056] An operating program embodying the fifth or sixth aspect of
the present invention may be provided by itself or may be carried
by a carrier. The carrier may be a recording medium such as a disk
or CD-ROM or may be a transmission medium such as a signal. The
appended claims are to be interpreted accordingly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] FIG. 1, discussed hereinbefore, shows parts of a wireless
communication system employing a HSDPA technique for downlink
transmissions;
[0058] FIG. 2, also discussed hereinbefore, shows an example of the
operation of the HSDPA technique in the FIG. 1 system;
[0059] FIG. 3, also discussed hereinbefore, is a graph illustrating
an example variation in signal-to-interference ratio of a downlink
channel over a series of transmission time intervals for four
different UEs in a wireless communication system;
[0060] FIG. 4, also discussed hereinbefore, is a graph for use in
explaining an adaptive modulation and coding technique;
[0061] FIG. 5, also discussed hereinbefore, shows an example of a
CQI mapping table;
[0062] FIG. 6, also discussed hereinbefore, is a schematic diagram
for use in explaining an automatic repeat request process;
[0063] FIG. 7, also discussed hereinbefore, is a graph
corresponding to FIG. 4 for illustrating how a UE speed affects
operation of an adaptive modulation and coding technique;
[0064] FIG. 8, also discussed hereinbefore, is another graph for
illustrating how different path conditions affect the operation of
an adaptive modulation and coding technique;
[0065] FIG. 9 is a schematic view of parts of a wireless
communication system for explaining signalling used therein;
[0066] FIG. 10 is a schematic view of a table employed in a first
embodiment of the present invention;
[0067] FIG. 11 is a flowchart for use in explaining an AMC method
according to a first embodiment of the present invention;
[0068] FIG. 12 is a timing diagram relating to the operation of the
first embodiment;
[0069] FIG. 13 is a schematic representation of a simulation model
used for simulating performance of an embodiment of the present
invention;
[0070] FIGS. 14 to 17 are graphs illustrating throughput versus
channel quality characteristics for comparing operation of an AMC
method embodying the present invention with conventional methods
under different UE speed and path conditions;
[0071] FIG. 18 is a flowchart for use in explaining an AMC method
according to a second embodiment of the present invention; and
[0072] FIG. 19 is a schematic representation of an AMC apparatus
according to a third embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0073] Before describing embodiments of the invention, reference is
made to FIG. 9 which is a schematic view for explaining signalling
in an HSDPA system.
[0074] For downlink signalling, three channels are used. A common
pilot channel (CPICH) is used to broadcast a signal to all UEs in
the cell served by the base station, in order to enable each UE to
measure a downlink channel quality based on the CPICH signal. A
high-speed downlink shared channel HS-DSCH is used to transmit
packet data to a UE. A high-speed shared control channel HS-SCCH is
used to carry transport format and resource related information
(TFIR). This TFIR is, for example, 8 bits and includes information
regarding a channelisation code, a MCS level, and a transport block
size. The HS-SCCH also carries HARQ related information. This HARQ
information is, for example, 12 bits and includes a HARQ process
number, a redundancy version, a new data indicator, and a UE
ID.
[0075] Uplink signalling is carried out using a high-speed
dedicated physical control channel (feedback channel) HS-DPCCH.
This channel is used to transmit a channel quality indicator (CQI)
value and an HARQ acknowledgement (ACK/NACK).
[0076] An AMC method according to a first embodiment of the present
invention will now be explained with reference to FIGS. 10 to 12.
This embodiment is used to adapt the MCS level of a downlink packet
access signal in an HSDPA system. Thus, in this embodiment the
transmitter is part of the base station and the receiver is part of
the user equipment.
[0077] In the first embodiment the base station maintains, for each
UE in its cell, a table of so-called "soft" MCS values. An example
of the soft MCS value table is shown in FIG. 10. The table 10 has
an upper row 12 and a lower row 14. The table is also divided into
a QPSK region 16 made up of the first 16 columns of the table, and
a 16QAM region 18 made up of the remaining 7 columns of the
table.
[0078] The upper row 12 of the table contains the set of available
CQI values. These CQI values correspond to the values 0 to 22
described previously with reference to FIG. 5. In this embodiment,
CQI values 23 to 30 are not available.
[0079] For each CQI value in the upper row 12, there is a
corresponding adjustable MCS value in the lower row 14. For
example, in FIG. 10 the soft MCS value 15.22 corresponds to the CQI
value 16.
[0080] The soft MCS values in the lower row 14 are adjustable in
use of the base station in dependence upon the channel conditions
experienced by the UE, as will now be explained with reference to
FIG. 11.
[0081] When a UE joins the cell served by the base station, in step
S1 a table of soft MCS values is created at the base station for
the joining UE. The soft MCS values in the lower row 14 are
initially set equal respectively to the corresponding CQI values in
the upper row 12. After the initialisation step S1 is completed,
the AMC method according to the first embodiment operates on a
frame-by-frame basis, and in each downlink frame (TTI of 2 ms)
steps S2 to S8 are carried out. Incidentally, the 3GPP
specifications also refer to a sub-frame as a period of 3 time
slots (2 ms). In this case, steps S2 to S8 can be carried out per
sub-frame.
[0082] In step S2, the UE produces a measure of downlink channel
quality for the latest frame. This measure is, for example, based
on the CPICH and represents a ratio of a received power .sub.or of
the CPICH signal to background noise including interference
I.sub.oc. The ratio .sub.or/I.sub.oc is a signal-to-interference
ratio.
[0083] Using an internal mapping table such as that described
previously with reference to FIG. 5, the UE identifies the highest
CQI value for which a single HS-DSCH sub-frame formatted with the
transport block size, number of HS-PDSCH codes and modulation
corresponding to the CQI value could be received with a transport
block error probability not exceeding a target value. For example,
the target transport block error probability may be 0.1.
[0084] For example, to identify the highest CQI value the measure
of downlink channel quality may be compared with a set of values
held by the UE for CQI value determination. There is one such
threshold value for each pair of adjacent CQI values. These
threshold values correspond to the threshold values Th01, Th02 and
Th03 described above with reference to FIG. 4. Based on the
comparison, the highest available CQI value at which the transport
block error probability target is achieved is identified.
[0085] Also in step S2 the UE carries out a cyclic redundancy check
(CRC) on the latest frame of the HS-DSCH signal. The CRC result
(pass or fail) is needed to generate the ACK/NACK message but, as
described below, it is also used for another purpose in the present
invention.
[0086] The CRC result and the CQI value are reported by the UE to
the base station using the HS-DPCCH.
[0087] In step S3, it is determined whether the reported CRC result
was a pass (ACK) or fail (NACK). If the CRC result is a pass,
processing proceeds to step S4. In step S4, each of the soft MCS
values in the QPSK region 16 of the table is increased by a first
upward adjustment amount .DELTA.UpQPSK. Similarly, each of the soft
MCS values in the 16QAM region 18 of the table is increased by a
second upward adjustment amount .DELTA.Up16QAM. Processing then
proceeds to step S6.
[0088] If, on the other hand, in step S3 the CRC result was a fail,
then in step S5 each of the soft MCS values in the QPSK region 16
of the table is decreased by a first downward adjustment amount
.DELTA.DownQPSK and each of the soft MCS values in the 16QAM region
18 of the table is decreased by a second downward adjustment amount
.DELTA.Down16QAM. Processing then proceeds to step S6.
[0089] In step S6, the CQI value reported by the UE in step S2 is
used as an index to the table 10 so as to identify the soft MCS
value corresponding to the reported CQI value. For example, as
shown in FIG. 10, the soft MCS value 15.22 corresponds to the CQI
value 16. In step S6 this soft MCS value is selected and rounded to
the nearest integer, which in this case is 15. This value is taken
as the next MCS level to be applied.
[0090] In step S7 it is checked whether the next MCS level is
within the permitted range of MCS levels. If the next MCS level
chosen in step S6 is lower than the lowest permitted MCS level then
the next MCS level is simply set to the lowest permitted MCS level.
Similarly, if the next MCS level selected in step S6 is higher than
the highest permitted MCS level then the next MCS level is set to
the highest permitted MCS level.
[0091] Finally, in step S8 the next MCS level determined in steps
S6 and S7 is applied to the downlink signal transmitted to the UE
in the next subframe.
[0092] FIG. 12 is a timing diagram for explaining the timing of the
operations in FIG. 11. As shown in FIG. 12, in a first subframe n
the UE receives data via the HS-DSCH using an initial or default
MCS level. The UE then has a period of 7.5 time slots in which to
process the data to produce the CRC result and the CQI value. Then,
in an uplink subframe k, the CRC result (ACK/NACK) and the CQI
value are reported to the base station via the HS-DPCCH.
[0093] The base station receives the reported ACK/NACK and CQI
value for subframe n, and determines the next MCS level before the
start of the next frame n+1. The base station then transmits the
next data to the UE via the HS-DSCH in the next HS-DSCH subframe
n+1 for that UE. As in the preceding frame, the UE has 7.5 time
slots to process the received data to produce the CRC result and
the CQI value. These are then reported back to the base station in
the HS-DPCCH subframe k+1, and so on for subsequent subframes.
[0094] In steps S4 and S5, .DELTA.Up16QAM may be set in dependence
upon a target packet error rate PER (transport block error
probability). For example, if the target PER is 0.1, 1 Up16QAM =
PER 1 - PER = 0.1 0.9 = 0.11 .
[0095] It was found empirically that suitable values for the other
adjustment amounts are:
.DELTA.Down16QAM=3*.DELTA.Up16QAM=0.33
.DELTA.UpQPSK=.DELTA.Up16QAM/10
.DELTA.DownQPSK=3*.DELTA.UpQPSK
[0096] Next, some simulation results will be described to show how
the performance of an AMC method embodying the present invention
compares with that of previously-proposed techniques. The first
previously-proposed technique which will be considered is the
adaptive threshold technique described in the introduction in which
the thresholds for switching between different MCS levels are
adjusted based on the ACK/NACK results (hereinafter "prior art
technique (1)"). The second previously-proposed technique is the
further technique described in the introduction in which the base
station selects a MCS level based on the ACK/NACK signalling from
the UE (hereinafter "prior art technique (2)").
[0097] The assumptions made in the simulations are set out in Table
1 below.
1TABLE 1 Parameters Values Propagation conditions 1-Path
Rayleigh/2-Path Rayleigh Fading Vehicle Speed for Fading 3,120 Kmph
CPICH power 10% of Tx Power at NodeB DSCH power 80% of Tx Power at
NodeB HSDPA frame Length 2 ms Spreading factor (SF) 16 or/Ioc
Variable MCS update 1 frame (2 ms) CPICH measurement delay 1 frame
MCS selection delay 1 frame CPICH measurement error Perfect Ch.
Estimation CPICH measurement report error rate Perfect Channel
Estimation Perfect Ch. Estimation Fast fading model Jakes spectrum
Channel coding Turbo coding with 22 MCS levels Tail bits 6 No. of
iterations for Turbo Coder 8 Metric for Turbo Coder Max Input to
Turbo Decoder Soft Number of Rake fingers Equals number of Paths
Hybrid ARQ None Information Bit Rates (Kbps) As shown in FIG. 5
Number of Multicodes Simulated As shown in FIG. 5 STTD Off
[0098] FIG. 13 is a schematic representation of the model used in
the simulations. In particular, it is assumed that the fading is
Rayleigh fading, the channel noise is Additive White Gaussian Noise
(AWGN), the receiver measures the channel quality
(.sub.or/I.sub.oc) perfectly, and the reporting of the CQI value
and CRC result is error-free.
[0099] FIGS. 14 to 17 each show a throughput versus downlink
channel quality characteristic for an AMC method embodying the
present invention (solid line), the prior art technique (1)
(dot-dash line) and the prior art technique (2) (dashed line).
[0100] FIG. 14 assumes that the UE is moving at a low speed of 3
kph. It is also assumed that the path conditions prevailing between
the base station and the UE are such that there is a single
dominant path. This kind of path condition arises, for example, in
open countryside, as opposed to urban environments.
[0101] FIG. 15 shows the corresponding results for the three
techniques, again under single path conditions, but with the UE
moving at a high speed of 120 kph. It can be seen from FIGS. 14 and
15 that the three techniques have more or less comparable
performance under single-path conditions.
[0102] FIGS. 16 and 17 show results corresponding to those of FIGS.
14 and 15 but under two-equal-gain path conditions, as might
prevail in an urban environment where there are many reflectors
such as buildings. In FIG. 16, the UE is assumed to be moving at
the low speed of 3 kph, whereas in FIG. 17 the UE is assumed to be
moving at the high speed of 120 kph. It can be seen that under
two-equal-gain path conditions, a method embodying the present
invention significantly outperforms both the prior art techniques
(1) and (2). In particular, compared to the prior art technique (2)
a method embodying the present invention provides approximately
118% throughput improvement at a UE speed of 3 kph and 230%
throughput improvement at 120 kph when the received signal power to
interference and noise ratio is 20 dB (.sub.or/I.sub.oc=20 dB).
[0103] From simulations it is believed that the prior art technique
(1) tends to track the fading much more tightly than a method
embodying the present invention which means that the spread of the
distribution of selected MCS levels is larger in prior art
technique (1) than in an embodiment of the present invention. Also,
the mean selected MCS level in a method embodying the present
invention was higher than that of prior art technique (1) in the
two-path simulation at 120 kph with mean SINR of 25 dB, even though
the PER was the same, so that a greater throughput is achieved in
the embodiment.
[0104] In the first embodiment described with reference to FIGS. 10
to 12, the UE reports the CRC result and the CQI level to the base
station, and the base station holds the soft MCS values table,
updates the table based on the CRC result and decides the next MCS
level. However, it is not necessary for the soft MCS values to be
held or updated in the base station, nor is it necessary for the
next MCS level to be decided by the base station. It is possible
for these operations to be carried out in the UE, as will now be
described in relation to a second embodiment of the present
invention shown in FIG. 18.
[0105] In the second embodiment, the steps are the same as the
steps S1 to S8 of the first embodiment except for the step S2 which
is replaced by a step S12 and the step S8 which is replaced by a
step S18.
[0106] As in the first embodiment, in step S1 a soft MCS values
table is created when the UE joins the cell. In the second
embodiment, this table is created inside the UE, rather than in the
base station. In step S12, the UE produces the CRC result and a CQI
value based on the latest received packet. Instead of reporting
these to the base station at this stage, the UE itself carries out
the steps S3 to S7 to select the MCS level for the next frame.
Then, in step S18 the UE reports the selected MCS level and the CRC
result to the base station using the HS-DPCCH.
[0107] Incidentally, in order to avoid delay in the CRC result
reaching the base station, it is possible for the UE to report the
CRC result to the base station in step S12, prior to carrying out
the processing of steps S3 to S7.
[0108] In the first and second embodiments described above, the
soft MCS values are produced using a table of soft MCS values as
shown in FIG. 10. The use of such a table has some significant
advantages. Firstly, because the table stores the corresponding
soft MCS value for each CQI value, it is possible to adjust the
soft MCS values by different amounts, if desired. Thus, for
example, instead of having a single upward and a single downward
adjustment amount for all of the soft MCS values in the QPSK region
16 it would be possible to have individual adjustment amounts for
each such soft MCS value. Alternatively, it would be possible to
adjust only some of the soft MCS values in reaction to a particular
CQI value, and leave others unchanged. Also, because all the soft
MCS values are held in the table, retrieval and updating of the
values can be quick and efficient. This is important as the
processing power available may be limited, particularly in the case
in which the table is held in the UE.
[0109] Nonetheless, despite these advantages, the requirement to
hold the soft MCS values in table form may lead to an increased
memory requirement, especially given that the soft MCS values are
non-integer values. This disadvantage is overcome in a third
embodiment of the present invention shown in FIG. 19.
[0110] In FIG. 19, in place of the soft MCS values table, two
parameters OFFSET_QPSK and OFFSET.sub.--16QAM are held and updated
in respective offset units 22 and 24. The offset unit 22 has an
input connected to an output of a first selection switch 26. The
switch 26 has first and second inputs for receiving the first
upward adjustment amount .DELTA.UpQPSK and the first downward
adjustment amount .DELTA.DownQPSK respectively. The second offset
unit 24 has an input connected to an output of a second selection
switch 28. The switch 28 has first and second inputs connected for
receiving the second upward adjustment amount .DELTA.Up16QAM and
the second downward adjustment amount .DELTA.Down16QAM
respectively. Each of the switches is controlled by the CRC result
from the UE (ACK/NACK). In particular, each selection switch 26 and
28 selects its first input when the CRC result is a pass (ACK) and
selects its second input when the CRC result is a fail (NACK).
[0111] Each of the first and second offset units 22 and 24 also has
a RESET input and an output. The output of the first offset unit 22
is connected to a first input of a third selection switch 30. The
output of the second offset unit 24 is connected to a second input
of the switch 30. An output of the switch 30 is connected to input
of an adder 32.
[0112] A CQI value receiving unit 34 is provided for receiving the
latest CQI value produced by the UE. The latest received value is
output by the unit 34 to another input of the adder 32. The CQI
value receiving unit 34 also produces a control signal QPSK/16QAM
which controls the selection switch 30. For example, when the
latest CQI value held by the unit 34 is in the range from 0 to 15,
the control signal QPSK/16QAM causes the switch 30 to select its
first input, whereas when the latest CQI value is in the range from
16 to 22 the control signal QPSK/16QAM causes the selection switch
30 to select its second input.
[0113] An output of the adder 32 is supplied to an input of an MCS
level range check/limit unit 36. The unit 36 outputs the next MCS
level.
[0114] Operation of the third embodiment shown in FIG. 19 will now
be described.
[0115] Firstly, when the UE joins the cell, the RESET inputs of the
first and second offset units 22 and 24 are activated so that the
parameters OFFSET_QPSK and OFFSET.sub.--16QAM are both reset to
0.
[0116] Then, in each frame, the CRC result is used to control the
selection switches 26 and 28 so that either the upward adjustment
amounts or the downward adjustment amounts are delivered to the
inputs of the first and second offset units 22 and 24. Each offset
unit 22 or 24 adds the received adjustment amount to the parameter
OFFSET_QPSK or OFFSET.sub.--16QAM it holds. Note that the downward
adjustment amounts are negative values in this embodiment.
[0117] Once the CQI value for the latest frame has been calculated,
this is received in the CQI value receiving unit 34. In dependence
upon the received value, the CQI value receiving unit 34 generates
the appropriate control signal QPSK/16QAM to control the selection
switch 30. Accordingly, the adder 32 either outputs the CQI value
plus OFFSET_QPSK or the CQI value plus OFFSET.sub.--16QAM. The MCS
level range check/limit unit 36 checks whether the output value
from the adder is within the permitted range (as in step S7 of FIG.
11), limits the output value as appropriate, and outputs the value
as the next MCS level.
[0118] Thus, in the third embodiment, even though there is no table
of soft MCS values, each available MCS level still has a
corresponding adjustable value (the parameter OFFSET_QPSK
corresponding to MCS levels 0 to 15, or the parameter
OFFSET.sub.--16QAM corresponding to MCS levels 16 to 22).
[0119] Although the embodiments described above have referred to
only two types of modulation scheme, namely QPSK and 16QAM, by way
of example, it will be appreciated that embodiments of the present
invention can be used with any suitable modulation schemes,
including eight phase shift keying (8PSK) and 64 quadrature
amplitude modulation (64 QAM). The soft MCS values table can have
as many regions as there are different modulation types.
Alternatively, in the FIG. 23 embodiment, there can be as many
offset units as there are different modulation types.
[0120] In the embodiments described above the transmitter was part
of the base station and the receiver was part of the user
equipment. However, in future networks it is likely that the user
equipment will be capable of applying an AMC method to the uplink
signals it transmits to the base station, in which case the methods
of any of the preceding embodiments can be carried out with the
transmitter being part of the user equipment and the receiver being
part of the base station.
[0121] Although an example of the present invention has been
described in relation to a wideband CDMA network having an
asynchronous packet mode, it will be appreciated that the present
invention can be applied to any other networks in which AMC can be
used. These networks could be, or could be adapted from, other CDMA
networks such as an IS95 network. These networks could also be, or
be adapted from, other mobile communication networks not using
CDMA, for example networks using one or more of the following
multiple-access techniques: time-division multiple access (TDMA),
wavelength-division multiple access (WDMA), frequency-division
multiple access (FDMA) and space-division multiple-access
(SDMA).
[0122] Those skilled in the art will appreciate that a
microprocessor or digital signal processor (DSP) may be used in
practice to implement some or all of the functions of the base
station and/or user equipment in embodiments of the present
invention.
* * * * *